1. Field of the Invention
This invention relates to a novel apparatus for quickly releasing the air from buoyancy tanks or housings in those situations when a quick deballasting of the buoyancy tanks or housings is required. A typical situation where this might occur is the case of a free-standing drilling riser that replaces most of the rig tension with positive buoyancy provided by buoyancy tanks or housings mounted on the riser sections. In the event of a catastrophic riser failure, i.e., where the riser below the buoyancy tanks or housings has parted or a bottom release in a dynamically positioned vessel drive off, the positively buoyant riser sections with attached buoyancy tanks or housings would ascend to the surface with increasing velocity to impact the drilling rig's or ship's hull with sufficient force to seriously damage the drilling rig. In the extreme case, the impact of the riser and buoyancy tank or buoyancy housing could sufficiently damage the drilling rig or ship to cause it to sink and lives to be lost.
The use of such positively buoyant risers with buoyancy tanks or housings providing the buoyancy is well known to those of ordinary skill in the art. Typical use dictates several riser sections will have buoyancy tanks or housings attached, depending on the water depth, to leave the riser disconnected and free standing or at least minimize the load on the rig's tensioner system while connected. The buoyancy tanks or housings are usually constructed with an open lower end to facilitate filling of the buoyancy tanks or housings by compressed air or other suitable gas. The volume and pressure of the supplied compressed air is determined by the water depth in which the buoyancy tanks or housings are to be used. Various methods of filling the buoyancy tank or housings either individually or in groups are well known to those of ordinary skill in the art.
Once the riser sections with attached buoyancy tanks or housings are in place the present invention has particular applicability. In the event the riser should fail as noted above, a particularly hazardous situation is presented by the positively buoyant tanks or housings. The present invention minimizes this hazardous situation by allowing a means to vent or deballast the buoyancy tank or housings in a few seconds. It is the rapid venting or deballasting of these buoyancy tanks or housings to which the present invention most closely applies.
2. Description of Related Art
A marine riser with open bottom air cans is shown in U.S. Pat. No. 4,099,560 (Fischer et al.) The apparatus shown by Fischer et al. discloses an air dump valve attached to a tether line that is activated in the event of the riser parting.
U.S. Pat. No. 4,176,986 to Taft et al. discloses another type of riser system with buoyancy tanks attached. A dump valve for rapidly venting the compressed air and controlled by a pilot valve assembly is shown.
Another marine riser with buoyancy system is disclosed in U.S. Pat. No. 4,422,801 (Hale et al.) The system shown by Hale et al. uses a quarter turn ball valve actuated by a trigger cable and air cylinder to vent the buoyancy air tanks.
U.S. Pat. No. 4,646,840 to Bartholomew et al., owned by the assignee of the current invention, discloses a buoyancy tank or housing system with a cascading system for supplying air to the buoyancy tank or housings.
All these systems are too slow for a dynamically positioned vessel that must vent in less than thirty seconds to avoid damage to the drilling vessel.